1. Field of the Invention
The invention generally pertains to electronic power conversion circuits. More particularly, the invention relates to a device for generating phase shift modulation for full-bridge converters, such as resonant converters.
2. Description of the Prior Art
Phase shift modulation is an alternative to pulse width modulation (PWM) for controlling full-bridge converters. PWM is a well-known control method widely used in power converters for many decades, therefore many PWM control circuits exist. Because phase shift modulation is quite new, however, there are only a few integrated circuits for generating this type of modulation. These integrated circuits mostly use phase shift modulators made of a combination of analogue and digital circuitry similar to those used in PWM generators. The generator is based on an analogue sawtooth oscillator. Its output is compared to the control signal that represents the requested phase and then the result is digitally processed to create two pairs of phase shifted signals for each half bridge in a full-bridge converter.
It is a simple method but it has several drawbacks. This type of generator cannot work in the full range of phases. It is difficult to generate phases close to zero and 180 degrees, so the range of allowable phases is narrower, there are some discontinuities at the edges of the range, or both.
A second method of generating phase shift modulation uses a standard analogue or digital PWM controller, wherein the PWM output is converted to phase shift modulated signals. The conversion may be performed by D flip-flop and simple combinatorial logic, for example. However, this method has the same drawback as the first method—the limited range of generated phase because it is difficult to convert PWM with a duty cycle approaching 0 or 100%. The second drawback of both mentioned methods is that they have problems with real time frequency tuning. For example, the frequency of generators based on analogue oscillators is set by passive components, so it is difficult to change in real time. Generators based on a digital PWM modulator have the ability to change the frequency in real time, but the frequency can only be changed in quite large steps. This lack of frequency tuning excludes these methods from being used for controlling resonant converters where smooth tuning of frequency is crucial. Also dead times, that are inserted between switch-off of one transistor and switch-on of the opposite transistor in the half bridge, are generated by analogue circuits. This means that dead times are set by passive components and cannot be changed in real time. Due to all these drawbacks, it is impossible to tune the converter to the optimum performance especially when soft switching at zero voltage and/or current is used.